“You either die a hero, or you live long enough to see yourself become the villain.”
Harvey Dent nailed it there: heroes and vigilantes may start out with good intentions, but they don't always stay on the path. The line certainly works for a character like Batman who skirts the law with everything he does, but what about other characters normally considered to be on the side of good?
Not everyone dresses in black and beats up bad guys in the name of what's right; many heroes stand tall and proudly defend the innocent from evil. As time goes on, heroes struggle with loss, pain, and being blamed for collateral damage among other things that take their toll. It can be tough being good when being bad is so much easier and there have been quite a few good guys who decided to dip their toes into the pool of evil.
These forays into the dark side of things generally last only a brief time, but for some, it could be the struggle that finally brings about their downfall. Whatever their reasons, even if they are completely beyond a person’s control, the good guys don’t always stay good.
10. Theon Greyjoy
When it comes to someone like Theon Greyjoy, people either hate him or love that they hate him and it has everything to do with what this guy did. Sure, he was taken hostage as a kid by the Starks, but they treated him like one of their own children. He grew up alongside his “siblings” and even helped wage war in the name of the North… until he flipped the script and betrayed everyone around him.
Possessed by some strange loyalty to his horrid father, Theon captured Winterfell and fake-executed his two little brothers. Granted, he didn't kill his adoptive kin and instead slaughtered a couple of innocent farmboys, but that is only slightly less evil.
Just when it's starting to look like Theon won't reform, he's captured by Ramsay Bolton, who is arguably the most sadistic fiend in all of literature. He picks up the name "Meek," loses the family jewels, and learns the error of his ways via a strict regimen of torture and mental abuse.
Theon eventually redeems himself by helping Sansa escape Bolton, but his foray into evil was pretty costly and horrific, which just goes to show you shouldn't betray your adoptive family for your real one if you're a main character on Game of Thrones.